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48                                INCISORS OF RODENTS

the Toothed Whales the teeth are hornodont, as they are in
the frog and in most reptiles ; on the other hand., some of the
remarkable reptiles belonging to Professor Huxley's order of the
Anomodontia have distinct canines, and show other differentiations
in their teeth.
A second characteristic of the mammalian dentition is the
limited number of the teeth, which rarely exceeds fifty-four.
Here again the Toothed Whales are an exception, the number of
their teeth being as great as in many reptiles. In the Mammalia
the number of the teeth is fixed (excepting of course for ab-
normalities), while in reptiles there is frequently no precise
normal. Two regions may be distinguished in every tooth—•
the crown and the root ; the latter, as its name denotes, is
imbedded in the gum, while the crown is the freely-projecting
summit of the tooth. The varying proportions of these two
regions of the tooth enables us to divide teeth into two series—•
the brachyodont and the hypselodont ; in the latter the crown is
developed at the expense of the root, which is small; the
hypselodont tooth is one that grows from a persistent pulp or, at
any rate, one that is long open. Brachyodont teeth on the
contrary have narrow canals running into the dentine. The
primitive form of the tooth seems undoxibteclly to be a conical
single-rooted tooth, such as is now preserved in the Toothed
Whales and in the canine teeth of nearly all animals. The de-
velopment of the teeth, that is, the simple bell-shaped form of the
enamel organ, seems to go some way towards proving this ; but it is
quite another question whether we can fairly regard the Whales
as having retained this early form of tooth. In their case the
simplification, as is so often the case where organs are simplified,
seems to be rather degeneration than retention of primitive
characters. But this is a matter which must be deferred for the
present.
The incisor teeth are generally of simple structure and nearly
always single rooted. In the Kocleiits, in the extinct Tillo-
dontia and in Diprotpdont Marsupials, they have grown large, and,
as has been already stated, they increase in size continuously
from the growing pulp. These teeth have a layer of enamel
only on the anterior face, which keeps a sharp chisel-like edge
upon them by reason of the fact that the harder enamel is worn
away more slowly than the comparatively soft dentine. The